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Monday, November 22, 2010

Temporary Battery Bank (More)

I made up the battery cables and fished them through the conduit.

The conduit in the picture is temporary as the riser has not been built yet (still a ways off). The goal is to keep the wire run as short as possible, secure, yet easy to get at should they need replacement.
 The battery cables are made up from some old spools of size 2/0 wire that had been on the shelf for a while (these cables are TEMPORARY. The cable jacket is labeled "marine cable", but in fact is far from it. It is thick stranded, non-tinned. And there are no manufacturer markings on it). I have not calculated DC electrical power requirements (and may not know ever if I keep changing my mind on things lol) so there is no "ideal" size of battery cable that can be calculated. But one could argue "the bigger you can fit, the better" when it comes to battery cables, as bigger means less electrical resistance and more efficient power transfer. It seems the largest cable available is 4/0. At least that is the size that most fuse blocks and switches are designed to accommodate. So, with 2/0 I am already going somewhat "big".

Question: One or Two Battery Banks?

For a point of reference, the battery box can hold twelve standard 6 volt golf cart batteries (fewer if other sizes are chosen). Even compared to your typical modern production sailboat, that is a LOT of batteries. There are upsides and downsides to having so many batteries in one bank. Upsides: less wiring, less switch hardware for management, less complicated charging setup. Downsides: batteries are like apples, if you get one bad apple in the cart, it tends to ruin the rest of the apples. With multiple batteries a bank, should one battery fail to perform to specs, it will tend to "bring down" any other "good" ones. It is for the same reasons I mentioned in the last post about not mixing different size batteries, etc. Should one cell "go bad", it will change the "charge appearance" of the entire bank and overall efficiency will be lost, which tends to shorten the life of all batteries in the bank. Therefore, the more batteries in a bank, the greater the chance one will turn into a "bad apple".

So, being undecided at this point, I plan on allowing for the "option" of two battery banks from the main battery box. Now I thought the temporary 2" conduit would hold two pairs of 2/0 cable runs. Well, it was tough enough to fish one pair even though there seems plenty of room in the conduit. So I am thinking about running two conduit runs as there is room to to so. Maybe I will try 1 1/2" conduit. With that said...

Question: Split the +/- cables or keep them together?

A couple readers have commented on the pros and cons of separating the positive and negative cables. The primary concern is chafe between the positive and negative cables causing a short (that would be a bad thing (tm)). So the idea of separating the cables was proposed. But this can have some downsides...

WARNING: thar be physics here.

Electrical current running through a wire creates a magnetic field (at right angles to the direction of current). The strength of this field is proportional to the amount of current flowing. Arguably, a boat's battery cables carries the MOST current most often compared to other systems on the boat and therefore, creates the strongest local magnetic field(s). If you are at all concerned about the accuracy of your ship's compasses (which use the earth's magnetic field to determine direction) you want to minimize interference from locally created magnetic fields as they affect the accuracy of the compass.

Now, if you run the + and - cables side-by-side you minimize the magnetic field as each cable creates the same strength magnetic field, but opposite each other (same amount of current going in opposite directions). The two fields tend to cancel each other out. If the cables are separated, this cancellation effect becomes less. The farther apart you separate them, the larger the magnetic field you will create, which  is bad for your compass accuracy.

Since you can't have these thick cables occupy the same space, a side by side run won't give you full cancellation of the field. But the next best thing you can do is to twist the pair of cables (which is still sorta tough with 2/0 cable).

So there is some food for thought. Gee that was long winded. Sorry for the lecture.

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